The invention relates to switchable display devices, and particularly, although not exclusively, to switchable liquid crystal display devices using spatial dither (ie. pixel subdivision) to achieve different grey levels. Embodiments of the invention allow a display panel to be operated at different resolutions, while maintaining an optimum number of spatial dither grey levels in the lower resolution, without increasing the number of drivers required.
Some prior art documents will first be discussed.
xe2x80x9cNew bistable liquid-crystal twist cellxe2x80x9dxe2x80x94D. W. Berreman and W. R. Heffner; J. Appl. Phys. 1981, 52 (4), 3032xe2x80x94describes the Bistable Twisted Nematic (BTN) effect, where selection between two metastable states (strong twist, e.g. 360 degrees, and weak twist, e.g. 0 degrees) is achieved by reducing a voltage either abruptly or gradually after blanking to the homeotropic weak state. This effect has the disadvantage of not giving grey level control.
European Patent Publication No.0613116 describes addressing with very short address times by placing the short selection pulse at an optimised position in time after the blanking pulse (so there is a pause between the two). Two modes of addressing are described as very high and very low selection voltage pulses both generate the highly twisted state (black in thin cells between crossed polarisers) whereas intermediate pulse voltages generate the low twist (white) state. This patent is important for fast BTN addressing as required for high resolution passive video displays. It has the disadvantage of not giving grey level control. xe2x80x9cA Bistable Twisted Nematic (BTN) LCD Driven by a passive Matrix Addressingxe2x80x9dxe2x80x94-T. Tanaka, Y. Sato, A. Inoue, Y. Momose, H. Nomura, S. Iino; Proceedings of Asia Display ""95, 259xe2x80x94presents a first black and white BTN panel. The pixels of this panel are not capable of displaying intermediate grey levels.
The use of the BTN effect as described in the prior art does not allow the generation of intermediate grey levels within the pixel.
The following publications relate generally to bistable LC effects and provide background information:
G. D. Boyd, J. Cheng and P. D. T. Ngo, Appl. Phys. Lett. 36, 556 (1979)
N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980)
R. Barberi, M. Boix and G. Durand, Appl. Phys. Lett. 55, 2506 (1989)
Barberi and G. Durand, Liquid Crystals 10, 289 (1991) R. Barberi, M. Giocondo and G. Durand, Appl. Phys. Lett. 60, 1085 (1992)
M. Pfeiffer et al., SID 95 Digest, 706
W. Hartmann, Ferroelectrics 122, 1 (1991) discusses grey scale techniques for FLCs.
R. A. Martin et al. Journal of the SID 4/2, 1996 p65 describes an active matrix display used as a binary mode. In this case greyscale is achieved by an error diffusion technique, the display does not have two separate resolution modes.
In matrix displays, greyscale can either be achieved within the pixel element, if the used effect offers an analogue or other range of configurations of different brightness, or by subdividing the pixel temporally or spatially if the effect offers only a black and a white state (or a limited number of states).m This invention is concerned principally with spatial subdivision of pixels into subpixels (spatial dither). The more subpixels you have, the more column (data) and row (strobe, select) drivers you normally need, and the cost of the display increases. It is often advantageous (and common practice in cathode ray tube (CRT) monitors) to have the choice between different screen resolutions. If such different resolutions are achieved in spatial dither panels, where a higher resolution with less greyscale and a lower resolution with more greys are offered, the optimal patterning of the subpixels is different for the two resolutions. To achieve both patterns the display has to be subdivided into more subpixels than will ever be addressed independently at one time. More subcolumns and/or subrows exist than are addressed independently, and therefore several drivers are used identically.
According to the invention there is provided a display device comprising a plurality of picture elements arranged in rows and columns, each picture element being switchable between at least two states having different optical output characteristics, a plurality of row and column drivers, connected to said rows and columns respectively, for switching said picture elements between said states, wherein at least one switch is provided for switching at least one of said drivers between a first configuration in which the driver drives at least one row or column, and a second configuration, different from said first configuration, in which the driver drives at least two rows or columns.
The invention thus reduces the number, and therefore the manufacturing cost, of the drivers.
Each picture element may be switchable between only two states.
Said two states may correspond to the picture element being on and off.
In one embodiment of the invention, at least some picture elements emit or transmit light at a different wavelength to other picture elements.
In this case, some of said picture elements may emit or transmit red light, some emit or transmit green light, and some emit or transmit blue light.
In a further embodiment of the invention, said one switch is a row switch which switches at least one of said row drivers between said first and second configurations, and the display device also comprises at least one column switch for switching at least one of said column drivers between a third configuration in which the column driver drives at least one column, and a fourth configuration, different from said third configuration, in which the column driver drives at least two columns.
In a further embodiment of the invention, operation of said switch or switches allows the display device to operate in at least two resolutions, including a high resolution mode and a low resolution mode
In a further embodiment of the invention, the number of grey levels in the low resolution mode is higher than would be possible without said switch or switches, for a device also capable of operating in said high resolution mode.
In a further embodiment of the invention, said picture elements are arranged in an array of identical groups of picture elements, each group being formed by picture elements at the intersection of number of adjacent rows and columns of picture elements, and each group containing picture elements of different sizes.
Each such group of picture elements may be connected to an identical switch or set of switches.
The display device may be formed from a liquid crystal panel.
The liquid crystal panel may form a passively addressed array.
The display device may be a bistable liquid crystal display device for example a bistable twisted nematic liquid crystal display or ferroelectric liquid crystal diplay.
The invention allows operation of a panel in at least two different resolutions with optimised numbers of grey levels and a minimised number of line drivers.